Catonic urea/formaldehyde resins, their preparation and their use in the paper industry

ABSTRACT

Cationic urea/formaldehyde resins are obtainable by condensing urea and formaldehyde in a molar ratio of from 1:1.5 to 1:3 in the presence of polymers which contain not less than 1 mol % of polymerized vinylamine units and have Fikentscher K values of from 5 to 300, in an amount of from 5 to 50 g per mole of urea in the end product, where the mixture is first a) precondensed at a pH of from 8 to 14, then acidified and b) condensed at a pH of from 1 to 5 until gel formation begins, c) then from 0.3 to 1.5 moles of formaldehyde are added per mole of urea used, d) post-condensation is carried out and e) the resin solution is subsequently neutralized. The resins thus obtained are used in papermaking as assistants for increasing the dry and wet strength of paper and result in less dulling of the whiteness of the paper.

The present invention relates to cationic urea/formaldehyde resins whichcontain condensed polymers having vinylamine units, a process for theirpreparation and their use in papermaking as an agent for increasing thedry strength and wet strength of paper.

U.S. Pat. No. 3,275,605 discloses that urea and formaldehyde can becondensed in the presence of polyamines to give water-soluble resins.The reactants are first precondensed in the alkaline pH range, thencondensed in the acidic pH range until gel formation begins, thensubjected to post-condensation, for example with formaldehyde, andsubsequently neutralized. The products are suitable as agents forincreasing the wet strength of paper.

U.S. Pat. No. 3,752,781 discloses a process for the preparation ofcationic resins, in which urea is first reacted with polyalkyleneimines,for example polyethyleneimine, with elimination of ammonia to givepolyureas, the polyureas are then reacted with urea and the reactionproduct is methylolated. This gives a cationic resin in the form of anaqueous solution, which is neutralized by adding an acid. Products ofthis type have not been used to date in the paper industry because theyare expensive and not very effective and furthermore lead to pronouncedyellowing of the paper.

European Patent 0,123,1961discloses a process for the preparation ofwater-soluble cationic urea/formaldehyde resins, in which urea andformaldehyde are condensed in a molar ratio of from 1:1.5 to 1:3 in thepresence of polyamines, the mixture first (a) being precondensed at a pHof from 8 to 14 and then acidified and (b) condensed at a pH of from 1to 5 until gel formation begins, then (c) from 0.3 to 1.5 moles offormaldehyde being added per mole of urea used, (d) postcondensationbeing carried out and the resin solution subsequently being neutralized.From 5 to 50 g of polyethyleneimine which contains from 20 to 15,000polymerized ethyleneimine units are used as polyamines, per mole of ureain the end product. The water-soluble cationic urea/formaldehyde resinsthus obtained are used in papermaking as assistants for increasing thedry strength and wet strength of the paper. However, when the paper webscontaining these resins as strengtheners are dried, undesirableelimination of formaldehyde is observed. Formaldehyde elimination duringthe drying of paper treated with the urea/formaldehyde resins disclosedin U.S. Pat. No. 3,275,605 as wet strength agents is, on the other hand,far greater than in the case of papers which contain condensates ofEuropean Patent 0,123,196 as strengtheners.

It is an object of the present invention to provide cationicurea/formaldehyde resins which increase the dry and wet strength ofpaper and impart to these papers the property of lower formaldehydeemission during drying compared with the conventional condensates basedon urea and formaldehyde. The novel resins should furthermore ensureless dulling in papermaking.

We have found that this object is achieved, according to the invention,by cationic urea/formaldehyde resins which are obtainable by condensingurea and formaldehyde in a molar ratio of 1:1.5 to 1:3 in the presenceof polymers which contain not less than 1 mol % of polymerizedvinylamine units and have K values of from 5 to 300 (determinedaccording to H. Fikentscher in 5% strength aqueous sodium chloride,solution at 25° C. and at a polymer concentration of 1% by weight), inan amount of from 5 to 50 g, based on 1 mole of urea in the end product,where the mixture is first

a) precondensed at a pH of from 8 to 14, then acidified and

b) condensed at a pH of from 1 to 5 until gel formation begins,

c) then from 0.3 to 1.5 moles of formaldehyde are added per mole of ureaused,

d) post-condensation is carried out and

e) the resin solution is subsequently neutralized.

The resulting solutions of cationic urea/formaldehyde resins are used inpapermaking as assistants for increasing the dry and wet strength ofpaper.

Urea and formaldehyde are condensed in a molar ratio of from 1:1.5 to1:3 in the presence of polymers containing polymerized vinylamine units.

Polymers containing vinylamine units are known, cf. for example U.S.Pat. Nos. 4,421,602, 4,623,699 and 4,255,548 and EP-A-0 216 387.Suitable polymers containing polymerized vinylamine units areadvantageously prepared by hydrolysis of homo- and copolymers containingpolymerized N-vinylamide units. Such polymers contain the followingcharacteristic structures: ##STR1## where R and R¹ are each H or C₁ -C₆-alkyl.

For the preparation of compounds which exclusively contain structuralunits of the formula I, for example N-vinylformamide, N-vinylacetamide,N-vinyl-N-methylformamide, N-vinylpropionamide or N-vinylbutyramide ispolymerized. Hydrolysis of the compounds having structural elements ofthe formula I using an acid, eg. hydrochloric acid, sulfuric acid orphosphoric acid, or in the presence of a base, such as sodium hydroxidesolution or potassium hydroxide solution, with elimination of the--CO--R¹ group, gives the structure: ##STR2## where R is H or C₁ -C₆-alkyl.

In the case of 100% hydrolysis of all units of the structure I in thepolymer, polyvinylamine is obtained. If only partial hydrolysis iscarried out, the polymers prepared from the N-vinylamides contain unitsof structures I and II. A preferably used N-vinylamide is the compoundof the formula ##STR3## ie. N-vinylformamide. Partially hydrolyzedhomopolymers of compounds of the formula III are disclosed in theabovementioned U.S. Pat. No. 4,421,602. As stated above, 100% hydrolysismay be carried out, so that polyvinylamine is formed. Other suitablepolymers containing polymerized vinylamine units are compounds which areobtainable by copolymerization of

A) from 1 to 99, preferably from 10 to 90, mol % of N-vinylamides and

B) from 99 to 1 mol % of vinyl acetate, vinyl propionate, C₁ -C₄ -alkylvinyl ethers, ethylene, esters, nitriles or amides of acrylic acid ormethacrylic acid, N-vinylpyrrolidone or mixtures, and subsequenthydrolysis of the amide groups of the structural units I of the polymersto give amino groups of the structure II.

Copolymers of N-vinylformamide with the comonomers stated under B) arepreferred. Such copolymers are described in EP-A-0 216 387. In additionto N-vinyl-formamide, it is possible to use, for example,N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide,N-vinylpropionamide or N-vinylbutyramide as monomers of component A. Inthe hydrolysis of copolymers of N-vinylformamide and vinyl acetate orvinyl propionate, it is possible, depending on the reaction conditions,also to hydrolyze the polymerized monomer units B), so that, in the caseo f the last-mentioned comonomers, the copolymers then additionallycontain, for example, polymerized vinyl alcohol units. This hydrolysismay be carried out partially or completely.

The homo- and copolymers containing polymerized vinylamine units have Kvalues of from 5 to 300, preferably from 15 to 150. Particularlypreferred polymers are those which contain the structural units of theformula II where R is H and have K values of from 20 to 95.

The polymers containing structural units of the formula II are used inthe condensation of urea and formaldehyde in an amount of from 5 to 50g, preferably from 10 to 30 g, based on 1 mole of urea in the endproduct.

From 2.0 to 2.5 moles of formaldehyde are preferably used per mole ofurea in the preparation of the condensates. The aqueous resin solutionsare obtained by first preparing a mixture of the abovementionedcomponents. It is in the form of a clear alkaline solution.Precondensation of the mixture of urea, formaldehyde and polymerscontaining polymerized vinylamine units in stage a) . . . (sic) carriedout at a pH of from 8 to 14, preferably from 9 to 11. In order for areaction to begin, the reaction mixture is heated to 60°-100° C.,preferably 70°-90° C. The precondensation essentially depends on thetemperature of the reaction and takes from about 10 minutes to 2 hours.Thereafter, the aqueous solution of the precondensate is acidified andis brought to a pH of from 1 to 5 in process step b). The reactionsolution can be acidified using mineral acids or organic acids, forexample sulfuric acid, phosphoric acid, hydrochloric acid, formic acid,acetic acid, propionic acid, p-toluenesulfonic acid and benzenesulfonicacid.

In process stage b), the condensation is carried out at a pH of from 1to 5, preferably from 2 to 4.5, and at from 60° to 100° C., preferablyfrom 70° to 90° C. The condensation is continued until gel formation isobserved. Gel formation is detectable by virtue of the fact that thrombiare no longer formed at the stirrer, even at relatively high speeds;instead, the reaction product rises up the stirrer. From 15 minutes to 3hours are required for the condensation reaction, depending on thetemperature, the shorter reaction times being associated with the highertemperatures.

Once a gel has formed, formaldehyde is added to the condensate inprocess stage c), from 0.3 to 1.5 moles of formaldehyde being added permole of urea used, and in process stage d) post-condensation is carriedout at from 60° to 100° C., preferably from 70° to 90° C. Thepost-condensation takes from about 10 minutes to 5 hours, depending onthe temperature set. In process stage e), the resin solution isneutralized. The neutralization is carried out using, for example,sodium hydroxide solution, potassium hydroxide solution, ammonia,amines, sodium carbonate, potassium carbonate, sodium bicarbonate orammonium or potassium bicarbonate or a mixture of two or more bases.Sodium carbonate is preferably used for the neutralization. After theneutralization, the ready-to-use aqueous resin solution has a pH of from6.0 to 7.0.

Aqueous solutions of cationic urea/formaldehyde resins having a solidscontent of from 20 to 70, preferably from 25 to 45, % by weight areobtained in this manner. The solids content of the solutions can readilybe varied by using aqueous formaldehyde solutions of a differentconcentration or paraformaldehyde in the condensation. The neutralaqueous solutions of cationic urea/formaldehyde resins obtained in thecondensation can be used for papermaking directly or, if necessary,after dilution with water. It is also possible to obtain that (sic)resin itself in solid form by distilling off the solvent. The cationicurea/formaldehyde resins are infinitely dilutable with water.

Water-soluble cationic modified urea/formaldehyde resins which, in theform of a 25% strength by weight aqueous solution, have viscosities offrom 20 to 1500 mPa.s at 20° C. are obtained. The aqueous resinsolutions or the solid products obtained therefrom are used asassistants for increasing the dry and wet strength of paper inpapermaking. The resins, in the form of aqueous solutions, are added tothe paper stock prior to sheet formation or are applied to a preformedpaper sheet with the aid of a size press. The cationically modifiedurea/formaldehyde resin is applied in amounts of from 0.1 to 5,preferably from 0.5 to 2, % by weight (solids), based on dry fibers. Theaqueous resin solutions are preferably added to the paper stock prior tosheet formation. In all fibers used in papermaking, they increase thedry and wet strength of the paper. They are used, for example, inbleached and unbleached pulp, groundwood, waste paper, CTMP and TMP orin mixtures of paper stocks. The advantage of the cationically modifiedurea/formaldehyde resins over similar prior art agents is that they dullthe whiteness of the paper only slightly and exhibit a substantiallylower formaldehyde emission during drying of the paper sheet.

The K values were determined according to H. Fikentscher,Zellulosechemie, 13 (1932), 48-64 and 71-74, in 5% strength sodiumchloride solution at 25° C. and at a polymer concentration of 1% byweight; K=k. 10³. In the Examples which follow, parts and percentagesare by weight, unless stated otherwise.

EXAMPLE 1

565 parts (7.5 mol) of formaldehyde, in the form of a 40% strengthaqueous solution, 220 parts (3.7 mol) of urea and 190 parts of waterwere mixed with 39.6 parts of a 42% strength aqueous solution of apartially hydrolyzed homopolymer of N-vinylformamide which contained 26parts of vinylamine hydrogen sulfate units and 74 parts ofvinylformamide units, and the mixture was heated to 70° C. whilestirring. The K value of the nonhydrolyzed polyvinylformamide was 30 andthe K value of the hydrolyzed polymer was 28.

After the mixture had been stirred for 30 minutes at 70° C., formic acidwas added until the pH had been brought to 4.0. At this pH and at 70°C., the reaction mixture was condensed until gel formation began. Gelformation was detectable by virtue of the fact that the reaction mixturerose up the rotating stirrer. Incipient gel formation is associated witha very pronounced increase in the viscosity of the resin solution. Thecondensation took about 2 hours. After the main condensation, a mixtureof 137 parts (1.8 mol) of formaldehyde, in the form of a 40% strengthaqueous solution, 30 parts of water and 50 parts of methanol was added,and the pH was brought to 5.8 by adding 20% strength sodium carbonatesolution. At this pH, postcondensation was carried out for 1 hour at 70°C. The resin solution was then brought to a pH of 6.2. This gave anaqueous solution, having a solids content of about 30%, of aurea/formaldehyde resin modified with a copolymer containing vinylaminegroups and N-vinylamide groups. This aqueous solution of the resin had aviscosity of 86 mPa.s.

COMPARATIVE EXAMPLE

Using the method described in European Patent 0,123,196, 565 parts (7.5mol) of formaldehyde, in the form of a 40% strength aqueous solution,220 parts (3.7 mol) of urea and 190 parts of water were mixed with 50parts of a 50% strength solution of polyethyleneimime (sic) whichcontained 35 . . . (sic) of polymerized ethyleneimine units, and themixture was heated to 70° C. while stirring. After the mixture had beenstirred for 30 minutes at this temperature, the pH of the mixture wasbrought to 4.3 by adding formic acid. At this pH and at 70° C., thereaction mixture was condensed until gel formation began. Thecondensation took about 2 hours. After the main condensation, a mixtureof 151 parts (2.0 mol) of formaldehyde, in the form of a 50% strengthaqueous solution, 4 parts of ethylenediamine, 30 parts of water and 50parts of methanol was added and the pH was brought to 5.8 with 20%strength aqueous sodium carbonate solution. The reaction mixture wasthen subjected to postcondensation for 1 hour at 70° C. The pH of theaqueous solution of the urea/formaldehyde resin modified withpolyethyleneimine was brought to 6.2. An aqueous solution having asolids content of 35% was obtained.

EXAMPLE 2

Example 1 was repeated with the sole exception that a 39% strengthaqueous solution of a partially hydrolyzed N-vinylformamide polymerwhich contained 45 parts of vinylamine hydrogen sulfate units and 55parts of N-vinylformamide units was used. This gave a roughly 30%strength aqueous solution of a urea/formaldehyde resin modified with apartially hydrolyzed N-vinylformamide polymer. This aqueous solution ofthe resin had a viscosity of 130 mPa.s (measured at 20° C.; Brookfield20 rpm).

EXAMPLE 3

Example 1 was repeated with the sole exception that a 30% strengthaqueous solution of a polyvinylamine having a K value of 28 was used asthe compound containing polymerized vinylamine units. A 29% strengthaqueous solution of a urea/formaldehyde resin modified withpolyvinylamine was obtained, the said solution having a viscosity(Brookfield, 20 rpm) of 146 mPa.s at 20° C.

EXAMPLE 4

The aqueous resin solutions obtained according to Examples 1 to 3 andthe Comparative Example were tested as assistants in papermaking. Forthis purpose, a suspension of a bleached softwood sulfite pulp having aconsistency of 0.5%, a pH of 4.5 and a freeness of 30° SR (SchopperRiegler) was first prepared. The cationically modified urea/formaldehyderesins, in the form of the aqueous solutions, were then added to thisfiber suspension in an amount of 1% (solids), based on dry paper stock.Sheets measuring 283.5 cm² and having a basis weight of 80 g/m² wereproduced from this paper stock. The properties shown in the Table weremeasured for the sheets thus obtained.

The dry breaking length were (sic) determined according to DIN 53 112,Sheet 1, and the wet breaking length according to DIN 53 1122, Sheet 2.The whiteness of the paper sheets was determined using a reflectometer(Elrepho), in accordance with DIN 53 145. The formaldehyde emitted ineach case during drying of the sheets was collected in water anddetermined by colorimetry.

                                      TABLE                                       __________________________________________________________________________                                       Wet breaking                                                                  length after                                          Formaldehyde            aging of the                               Resin prepared                                                                           emission                                                                              Dry breaking                                                                          Wet breaking                                                                          paper (5 min,                                                                          Whiteness                         according to Example                                                                     μg/sheet                                                                           length in meters                                                                      length in meters                                                                      130° C.)in meters                                                               % R.sup.1)                        __________________________________________________________________________    1          119     2723    400     498      84.43                             2          144     2694    433     562      83.68                             3          146     2664    441     573      83.0                              Comparative Example                                                                      238     2736    483     681      82.58                             __________________________________________________________________________     .sup.1) R = Reflectance                                                  

We claim:
 1. A cationic urea/formaldehyde resin, produced by condensing urea and formaldehyde in a molar ratio of from 1:1.5 to 1:3 in the presence of polymers which contain not less than 1 mol % of polymerized vinylamine units and have K values of from 5 to 300 (determined according to H. Fikentscher in 5% strength aqueous sodium chloride solution at 25 ° C. and at a polymer concentration of 1% by weight), in an amount of from 5 to 50 g, based on one mole of urea in the end product, where the mixture is firsta) precondensed at a pH of from 8 to 14, then acidified and b) condensed at a pH of from 1 to 5 until gel formation begins, c) then from 0.3 to 1.5 moles of formaldehyde are added per mole of urea used, d) post-condensation is carried out and e) the resin solution is subsequently neutralized.
 2. A process for the preparation of a water-soluble cationic urea/formaldehyde resin, which comprises condensing urea and formaldehyde in a molar ratio of from 1:1.5 to 1:3 in the presence of polyamines, where the mixture is firsta) precondensed at a pH of from 8 to 14, then acidified and b) condensed at a pH of from 1 to 5 until gel formation begins, c) then from 0.3 to 1.5 moles of formaldehyde are added per mole of urea used, d) post-condensation is carried out and e) the resin solution is subsequently neutralized,wherein a polymer which contains not less than 1 mol % of polymerized vinylamine units and has a K value of from 5 to 300 (determined according to H. Fikentscher in 5% strength aqueous sodium chloride solution at 25° C. and at a polymer concentration of 1% by weight) is used as the polyamine, in an amount of from 5 to 50 g, based on 1 mole of urea in the end product.
 3. The cationic urea/formaldehyde resin of claim 1, wherein said polymerized vinylamine units have the formula: ##STR4## wherein R is H or C₁ -C₆ -alkyl.
 4. The cationic urea/formaldehyde resin of claim 1, having a K value of from 15 to
 150. 5. The cationic urea/formaldehyde resin of claim 3, which comprises structural units of the formula: ##STR5## wherein R is H, and which resin has a K value of from 20 to
 95. 6. The process as claimed in claim 2, wherein the polymer containing vinylamine units is obtained by:a) copolymerizing from 1-99 mol % of a N-vinylamide selected from the group consisting of monomer, N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinylpropionamide and N-vinylbutyramide; and from 99 to 1 mol % of vinyl acetate, vinyl propionate, C_(l) -C₄ -alkyl vinyl ethers, ethylene, esters, nitriles or amides of acrylic acid or methacrylic acid, N-vinylpyrrolidone or mixtures thereof, and b) subsequently hydrolyzing amide groups of polymerized units produced in step a) to produce amino groups.
 7. The process as claimed in claim 6, wherein from 10 to 90 mol % of said monomer is used.
 8. The process as claimed in claim 6, wherein in step a) a pH of from 9 to 11 is used.
 9. The process as claimed in claim 6, wherein in step a), said urea and formaldehyde are precondensed at a temperature of from 60° to 100° C.
 10. The process as claimed in claim 6, wherein in step b), said urea and formaldehyde are condensed at a pH of from 2 to 4.5.
 11. The process as claimed in claim 6, wherein in step b), said urea and said formaldehyde are condensed at a temperature of from 60° to 100° C.
 12. The process as claimed in claim 6, wherein in step d), said post-condensation is effected at a temperature of from 60° to 100° C. 